You can export HTML and XLIFF directly from the Drupal Translation Management Tool (TMGMT) without installing any additional plugins. Of course we recommend that you actually use our plugin that connects you directly to our Supertex API and gives you a quote without any copy&paste hassles:

It is not immediately clear to me how to work with an HTML export. Does one just replace the text that is there and re-upload? As opposed to XLIFF, where there is a separate element to put the translated text.